THE ACTION OF ONE CROP ON ANOTHER. 
373 
all growth practically ceased, and in the case of the weaker varieties, 
such as Cox, the trees were actually killed within two or three years. 
A similar result was obtained when the laying down to grass was post- 
poned till thirteen years after the trees had been planted : the planta- 
tions in this case contained trees of sundry varieties of apples and pears, 
grown both as standards and dwarfs, and, whilst the different varieties 
showed great differences as to the extent to which they were affected, 
the general results may be gathered from the fact that the value of 
the crops from the grassed section was, during the four succeeding 
years, only 41 per cent, of that from the sections which had remained 
ungrassed, and, as in the previous case, some of the trees were actually 
killed. Yet another experiment was made on a plantation of standard 
trees of Bramley's Seedling after they had been established for twenty- 
two years, being then remarkably fine specimens of this variety. The 
grass did not come up well, but, in spite of this, the trees in the grassed 
section showed the effect even in the early part of the summer of the 
first year, by a marked difference in the appearance of the foliage, 
clearly visible from a considerable distance, whilst, later in the season, 
the effect was still more apparent by the grassed trees losing their leaves 
two or three weeks earlier than the ungrassed ones. The crops, also, 
were found to have been adversely affected by the grass. 
Thus, the deleterious effect of grass in our soil is independent of 
the age of the trees when the grassing over is effected. 
The peculiar effect of grass on the colour of the foliage is paralleled 
by its effect on the colour of the bark and the fruit : this effect is 
distinctive, and, being unlike that produced by lack of water or 
nourishment, suggested from the first some toxic influence. As 
regards the fruit, a lack of colouring matter is evident in the case of 
some varieties, but in others the change consists of an alteration in 
the colour, the green giving place to a red pigment. Such a change 
may be beneficial from the point of view of the grower, and, provided 
the action is not excessive, grass may also benefit the grower by causing 
increased cropping ; for, as is well known, limited injury to the tree 
often results in increased productiveness. What is best for the tree 
is not always best for the grower. 
The injury to the tree affected by grass is not confined to the case 
of apple trees : precisely similar results have been obtained with every 
kind of tree investigated : pears, plums, cherries, oak, beech, ash, 
spruce, larch and fir, with the last three, even when they are grown 
in a light sandy soil best suited to their welfare. 
Our search for the explanation of the effect of grass on trees con- 
sisted, in the first place, of an examination and exclusion, one by one, 
of the various causes which might be suggested for the action. Of 
these, space will allow only of the mere enumeration of the majority: 
the alterations produced by grass on the aeration of the soil, the 
carbon dioxide content of the soil, the temperature of the soil, its 
possible alkalinity, its physical condition, and its bacterial condition ; 
and in all these cases negative results were obtained. A little more 
